1. Field of the Invention
This invention relates to an electrophotographic recording apparatus, and more particularly to an electrophotographic recording apparatus using a recording array head.
2. Description of the Related Art
An electrophotographic recording apparatus is of non-impact recording type and is advantageous over various other recording apparatuses in that the operation noise is small, characters can be clearly recorded, the recording speed is high, and the running cost is relatively low. Because of the above feature, recently, the electrophotographic recording apparatus is used as an output terminal device of an OA instrument and the market for the electrophotographic recording apparatuses is rapidly growing.
The outline of the electrophotographic recording apparatus is explained with reference to FIG. 1. FIG. 1 is a schematic view of the recording section of a laser printer which is one type of electrophotographic recording apparatus.
As shown in FIG. 1, the electrophotographic recording apparatus generally uses a photosensitive drum 200. The entire surface of the photosensitive drum 200 is uniformly charged by negative charges to approximately -700 V, for example, by use of a charging unit 201 of corona charging type. Next, a recording light 202 is applied to the surface of the photosensitive drum 200 according to an image signal. Since the resistance of only that portion of the photosensitive drum 200 which is illuminated is lowered, the negative charges on that portion of the photosensitive drum 200 to which the recording light 202 is applied are eliminated, thus making an electrostatic latent image. In this case, the recording light 202 is generally a laser light and the light modulated according to the image is scanned by a rotating polygon mirror. A laser for emitting the laser light is formed of one semiconductor laser.
The electrostatic latent image formed in the above-described manner is developed by a developing unit 203. The developing unit 203 attaches toners which are colored fine particles and negatively charged by the negative image development, for example, to that portion of the electrostatic latent image on the photosensitive drum 200 from which negative charges are removed by application of a development bias voltage of approximately -500 V, thereby converting the electrostatic latent image into a visible image.
Recording paper 205 taken out from a paper cassette (not shown) by a paper supply roller 204 is fed in synchronism with an image signal and brought into contact with the photosensitive drum 200. When the recording paper 205 is set in contact with the photosensitive drum 200, the visible image which is converted from the electrostatic latent image is transferred to the recording paper 205. In this case, a transfer charger 206 generates positive charges towards the rear surface of the recording paper 205, for example, to attract the image developed by the negatively charged toners on the photosensitive drum 200 onto the recording paper 205, thus transferring the image to the recording paper. The recording paper 205 to which the image is transferred by the transfer charger 206 is separated from the photosensitive drum 200 by a separation charger 207. Finally, the toners are heated and pressed in a fixing unit 211 constructed by heating rollers 210 and thus fixed on the recording paper 205, and the recording process is completed.
Toners which are not transferred to the recording paper 205 are left behind on the photosensitive drum 200 after the image has been transferred. The remaining toners are scraped off by use of a cleaner constructed by a cleaning blade 208 to clean the photosensitive drum 200. After this, the entire surface of the photosensitive drum 200 is exposed to light from an erasing lamp 209 constructed by an LED or the like to remove the charges on the photosensitive drum 200.
As described above, in the electrophotographic recording apparatus, an image is formed by effecting the charging, latent image forming, transferring and fixing steps. The photosensitive drum 200 is cleaned in the cleaning step to be used for the next recording process.
The units used in the respective steps may be somewhat different in construction in different recording apparatuses, but have basically the same constructions as described above.
The laser printer has been explained above as a typical example of the electrophotographic recording apparatus. As the electrophotographic recording apparatus, not only the laser printer but also various apparatuses having different types of light emitting elements as recording heads for forming the electrostatic latent image are developed and manufactured. In the laser printer, image dots are scanned by light emitted from a single laser by use of a polygon mirror which is mechanically rotated at high speed or hologram, but at present, a fixed scanning type apparatus using an array-form light source starts to receive much attention by taking the small size and low price of the apparatus into consideration. For example, electrophotographic recording apparatuses using recording heads having optical shutter elements and light emitting elements such as fluorescent materials, LEDs, liquid crystal shutters, EL elements, and plasma light emitting elements which are arranged in an array form have been developed and put to practical use. The above recording heads are different in the recording method, but the driving/controlling methods for the recording elements are based on substantially the same principle. That is, the resolution of the recording elements is set to coincide with that of the image signal and a binary recording signal corresponding to the image signal is supplied to the recording elements to selectively drive necessary recording elements.
Each of the above electrophotographic recording apparatuses is called an optical printer and is used as an output device of a digital copying machine or a printer. An analog copying machine which copies an original by applying light from a fluorescent lamp or the like to the original and guiding the reflected light to a photosensitive body to form an electrostatic latent image is also one type of electrophotographic recording apparatus.
As described above the electrophotographic recording apparatus has excellent features and is widely used as the output terminal device of an OA instrument, various systems have been developed, and the market thereof is rapidly growing.
The electrophotographic recording system is roughly divided into two recording systems. The first recording system is used to record an image of one line by creating an image recording signal by use of one element and scanning the element in a main scanning direction (which is the direction along the rotation axis of the photosensitive drum 200, the main scanning direction is thus defined in the following explanation and the rotation direction of the photosensitive drum 200, that is, a direction perpendicular to the main scanning direction is defined as a sub-scanning direction), and a laser printer for scanning a laser light by use of a polygon mirror is a typical example of this system. On the other hand, the second recording system is a recording system which is called a fixed scanning system for creating an image of one line by arranging a large number of light emitting elements and optical shutters on a line (in an array form) and individually controlling the elements.
The feature of the first recording system is that all of the image dots are created by light emitted from a single light source and therefore there occurs no variation in the density of the image dots. Therefore, this system is extremely suitable for recording a graduation image. However, since the entire surface of the recording paper is scanned by the same light, it takes a long time for the scanning operation, and in most cases of the apparatuses actually used, the recording speed is approximately 6 pages/min. In order to attain the higher scanning operation speed, a motor for rotating the rotating polygon mirror and motor bearings must be specially and precisely designed.
In contrast, in the fixed scanning type system, since one line can be simultaneously scanned, it is easy to enhance the recording speed. Further, since it is not necessary to provide a long optical path for scanning, the size can be reduced. Since the recording head is disposed in the close vicinity of a recording medium, it is easy to precisely set the head to a desired position. Thus, the array-form recording head has some advantages which the laser printer does not have.
However, some problems caused by use of a large number of elements occur.
The first problem is that since a large number of elements, for example, 3360 dot elements in the case of a recording head having the A4-width of 16 dots/mm are arranged on one line, the light emission amount and light transmission amount of the elements may be different from one another so that the densities of the recording image dots may vary. The variation is significantly different for each system, but it is approximately 30% in the LED array head, and when a half-tone image is recorded, difference between outputs of the respective elements appear as noise, thereby degrading the image quality. If the variation is 15 to 30%, no substantial influence will occur in the case of recording of binary information such as characters. However, in the recording of graduation image or full color image, high density portions and low density portions appear in a stripe form or the same color appears different in different positions. As a method for compensating for this variation, a method of compensating for the variation by separately recording one pixel in a plurality of cycles and effecting the pulse width modulation is proposed, but in this case, the variation can be reduced only to approximately 10%.
The second problem is that when an oblique line is recorded, notches whose magnitude corresponds to the resolution of the recording head appear on the contour. Further, when a multivalue image is recorded, a plurality of elements must be used in the dither method so that the resolution may be lowered and the texture of the dither will appear as noise.